With the realization of high-speed and highly integrated semiconductor devices, metal lines formed within the semiconductor devices are getting finer with multi layers. In such a case, the widths of the metal lines are decreased as well, thereby causing a signal delay due to the resistance and capacitance of the metal lines. Thus, to reduce such a signal delay, copper having a low resistance has been widely employed for the formation of the metal line.
In contrast to conventional metals employed, copper is difficult to etch. Accordingly, a copper wiring is formed through a damascene process including the steps of: forming a trench first; then forming a copper layer to fill the trench; and finally performing a chemical mechanical polishing thereon.
Since copper tends to diffuse into other layers easily, a barrier layer is formed on the trench before filling the trench with copper.
Though the barrier layer can be formed of Ta film, a Ta film cannot prevent the diffusion of copper perfectly. For this reason, the barrier layer has been formed of TaN. However, the TaN film is disadvantageous in that its adhesive strength with copper is low, though it can prevent the diffusion of copper more effectively than the Ta film.
Thus, nowadays, the barrier layer is formed of a dual film of Ta/TaN, so as to improve the reliability of the semiconductor device. The dual-film barrier layer can be formed by using a physical vapor deposition (PVD) process or an atomic layer deposition (ALD) process.
However, when the dual-film barrier layer is formed by the PVD process, an overhanging phenomenon may occur, in which an entrance of a via is blocked, if the aspect ratio (a ratio of a depth to a width) of the via is large, which results in a failure to form a barrier layer appropriately.
Meanwhile, when forming the dual-film barrier layer by the ALD method, a TaN film and a Ta film are formed by using different precursors, so the whole process becomes complicated. Furthermore, since carbon (C) and oxygen (O) are contained in the obtained TaN film, the resistivity of the TaN film is increased.